(1) Field of the Invention
The present invention relates to a doubly resonant push-pull class IV (dog bone) flextensional transducer having an improved operational bandwidth.
(2) Prior Art
Flextensional transducer devices are known in the prior art and have been used in a wide variety of applications. For example, U.S. Pat. No. 3,583,677 to Phillips describes an electro-mechanical transducer for secondary oil recovery. The transducer provides a dipole-type radiation field which extends along a single axis perpendicular to the axis of an oil well. This allows a surrounding casing to vibrate in a displacement mode rather than in a circumferential expansion mode to enable energy coupling to a surrounding oil-producing formation. The transducer includes two resonant beams forced to vibrate at an audio or sonic frequency by piezoelectric element stacks driven by an external electrical power source and transferring energy through additive shear waves to an external body. The transducer described in Phillips is a single frequency device operating in a shear bending mode.
U.S. Pat. No. 4,462,093 to Upton illustrates a transducer support system that couples the weight of the active portion of a transducer to the transducer's flanges without coupling the dynamic motion of the active portion of the transducer to the transducer's flanges. This support system is used in a flextensional transducer to produce a transducer with increased acoustic output and lower frequency.
U.S. Pat. No. 4,764,907 to Dahlstrom et al. illustrates an underwater transducer which includes a centrally located beam, a plurality of stacks of piezoelectric transducer elements extending from each side, and a rigid end beam at the opposite end of each stack. A plurality of bolts extending from one end beam to the other, on opposite sides of the stacks, are tightened to apply a desired amount of prestress on the ceramic stacks. Arcuate radiating elements are welded to opposite sides of each end beam, end cap members are fastened to the centrally located beam at each end of the transducer, and a jacket of elastomeric material is bonded to the edges of the end cap members to prevent ingress of fluid into the piezoelectric elements. Energizing of the piezoelectric elements causes expansion and contraction of the stacks, pushing the end beams in and out and causing bowing of the radiating elements to project sonar energy.
U.S. Pat. No. 5,291,461 to Boeglin et al. relates to an elastomer support for a sonar transducer which includes a ceramic stack electromechanical driver, a pair of rigid support members, and a pair of elastomer layers disposed between the ceramic stack electromechanical driver and the support members. The elastomer support provides effective mechanical stress reduction in the ceramic stack driver. The Boeglin et al. patent also describes a technique for removing excess heat from the ceramic stack components under high drive conditions.
U.S. Pat. No. 5,566,132 to Janus et al. relates to an acoustic transducer to which mechanical pre-stress is applied to the piezoceramic by spreading two plates symmetrically from the center with a bolt and nut arrangement. The acoustic transducer includes a housing and first and second stacks of transduction plates disposed within the housing. The first and second stacks are adapted to be held in compression between the housing opposed wall portions. A threaded stud extends from the first stack to the second stack. A nut is threadly engaged with the stud and adjacent one of the stacks. Movement of the nut on the stud operates to move the stacks toward the housing walls to compress the stacks, and operates to relax compressive force on the stacks to enable withdrawal of one of the stacks and replacement thereof.
Recently, polycrystalline and single crystal electrostrictive drive materials have been receiving considerable interest in research programs due to their significantly higher output power density potential. However, direct substitution of these new materials for those currently used is not possible without redesigning the transducer element's mechanical and/or electrical configuration. This is due to both elastic moduli differences and the fact that the new materials, unlike their PET counterparts, come unpolarized and require a D.C. bias for linear operation. One method of eliminating the need for D.C. bias for linear operation is operating the drive material in a push-pull mode whereby half-stack pairs are driven unidirectionally with alternating and opposing polarity.